My sample had been disassembled completely before it was sold just to find
out what caused it. Many Pentax K mount manual focus fixed focal length
lenses were designed and built the same way with 2 metal (some plastic)
helicoid guide plates guiding the position of the whole focusing mechanisms.
These 2 plates were often placed at both sides of the lens so that they are
perfectly symmetric. However, in the case of the 77, due to the very limited
space (and the filter ring design shows) of this design, one of the plates
was forced to be placed so that they are not symmetric. Because of this, the
weight of the whole lens group (quite heavy) rests on these 2 metal plates
vary depends on the physical position of the lens. To see the real
difference, just mount a heavy metal hood on it (say 49mm thread 100mm lens
hood) and try to manual focus slowly in landscape orientation. You can
actually feel the resistance which might make you uncomfortable.
Unfortunately, since this is an AF lens, heavy damping grease which might
ease the friction, cannot be used. Another con of this 2 guiding plates
design is that the filter thread section might become loose over time
(rotate slightly), especially when filters were forced mounted on them, and
quite common on many 'A' lenses due to their loose tolerance. I would be
very interested to see how Zeiss, Leica or Nikkor lenses were designed and
built, but unfortunately, I can't afford any. <g>
Regards,
Alan Chan
http://www.pbase.com/wlachan
My question to you, the resident disassembler :-).
Do you know why this happens? Obviously, it does not harm the lens, but
what part of lens internal is giving this resistance due to a different
orientation of the lens? Have you disassembled this lens before?
Something
must shift inside the lens when the lens is rotated.
I am just curious.
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